A gel, particularly a hydrogel contains water as a main component, and therefore, has recently attracted attention from the viewpoint of a soft material having high biocompatibility and a low environmental load. As one example, a self-supporting organic-inorganic composite hydrogel that can be easily produced by mixing a polyelectrolyte, clay particles, and a dispersant at room temperature without a polymerization reaction has been reported (Non-Patent Document 1),
In order to enhance the mechanical strength of the hydrogel, the concentration of a solute or a dispersoid has to be increased. However, in the hydrogel described in Non-Patent Document 1, when the concentration of a solute or a dispersoid is increased, the viscosity is extremely increased, and the production of the gel is difficult. For example, a method for forming a gel by uniformly pouring a gel-forming composition in a flow state into a flat container during production of a sheet-shaped gel is desired. However, for the hydrogel produced from a polydectrolyte, clay particles, and a dispersant, when a composition having such flowability that the processing is easy is used, only a sheet-shaped gel having a low strength can be produced.
A dry clay film having a self-standing property (self-supporting property) that contains a polyacrylate salt and a clay mineral is known (Patent Document 1), and a gelled paste is produced as an intermediate. However, this paste does not have a self-supporting property seen in Non-Patent Document 1. Further, this paste is used only as a surface-protection material for purposes of water resistance and heat resistance by applying and drying the paste to form a film, followed by exchange with potassium ions to impart a function of preventing expansion and decomposition of the film that are caused by moisture.
As a method for producing a sheet-shaped hydrogel by drying, the formation of a sheet from polyvinyl alcohol (PVA) as a raw material is known (Patent Document 2). This method is a technique of increasing the strength by layering PVA gel sheets having a low strength. However, an operation of forming a multilayer is complicated.